Laboratory Microcosm Experiments Research Articles

Functional ecology of aquatic phagotrophic protists – Concepts, limitations, and perspectives

Functional ecology is a subdiscipline that aims to enable a mechanistic understanding of patterns and processes from the organismic to the ecosystem level. This paper addresses some main aspects of the process-oriented current knowledge on phagotrophic, i.e. heterotrophic and mixotrophic, protists in aquatic food webs. This is not an exhaustive review; rather, we focus on conceptual issues, in particular on the numerical and functional response of these organisms. We discuss the evolution of concepts and define parameters to evaluate predator–prey dynamics ranging from Lotka–Volterra to the Independent Response Model. Since protists have extremely versatile feeding modes, we explore if there are systematic differences related to their taxonomic affiliation and life strategies. We differentiate between intrinsic factors (nutritional history, acclimatisation) and extrinsic factors (temperature, food, turbulence) affecting feeding, growth, and survival of protist populations. We briefly consider intraspecific variability of some key parameters and constraints inherent in laboratory microcosm experiments. We then upscale the significance of phagotrophic protists in food webs to the ocean level. Finally, we discuss limitations of the mechanistic understanding of protist functional ecology resulting from principal unpredictability of nonlinear dynamics. We conclude by defining open questions and identifying perspectives for future research on functional ecology of aquatic phagotrophic protists.

Exogenous nutrients and carbon resource change the responses of soil organic matter decomposition and nitrogen immobilization to nitrogen deposition.

It is unclear whether exogenous nutrients and carbon (C) additions alter substrate immobilization to deposited nitrogen (N) during decomposition. In this study, we used laboratory microcosm experiments and 15N isotope tracer techniques with five different treatments including N addition, N+non-N nutrients addition, N+C addition, N+non-N nutrients+C addition and control, to investigate the coupling effects of non-N nutrients, C addition and N deposition on forest floor decomposition in subtropical China. The results indicated that N deposition inhibited soil organic matter and litter decomposition by 66% and 38%, respectively. Soil immobilized 15N following N addition was lowest among treatments. Litter 15N immobilized following N addition was significantly higher and lower than that of combined treatments during the early and late decomposition stage, respectively. Both soil and litter extractable mineral N were lower in combined treatments than in N addition treatment. Since soil N immobilization and litter N release were respectively enhanced and inhibited with elevated non-N nutrient and C resources, it can be speculated that the N leaching due to N deposition decreases with increasing nutrient and C resources. This study should advance our understanding of how forests responds the elevated N deposition.

The number of limiting resources in the environment controls the temporal diversity patterns in the algal benthos.

The role of the number of limiting resources (NLR) on species richness has been the subject of much theoretical and experimental work. However, how the NLR controls temporal beta diversity and the processes of community assembly is not well understood. To address this knowledge gap, we initiated a series of laboratory microcosm experiments, exposing periphyton communities to a gradient of NLR from 0 to 3, generated by supplementation with nitrogen, phosphorus, iron, and all their combinations. We hypothesized that similarly to alpha diversity, shown to decrease with the NLR in benthic algae, temporal beta diversity would also decline due to filtering. Additionally, we predicted that the NLR would also affect turnover and community nestedness, which would show opposing responses. Indeed, as the NLR increased, temporal beta diversity decreased; turnover, indicative of competition, decreased; and nestedness, suggestive of complementarity, increased. Finally, the NLR determined the role of deterministic versus stochastic processes in community assembly, showing respectively an increasing and a decreasing trend. These results imply that the NLR has a much greater, yet still unappreciated influence on producer communities, constraining not only alpha diversity but also temporal dynamics and community assembly.

Effects of various bivalves on meiobenthic and nematode assemblages in shallow sandy sediments

A laboratory microcosm experiment was performed to study the effect of bivalves on meiobenthic and nematode community structures. Three bivalve species dominant in shallow coastal sediments of the southern Baltic Sea differing in terms of feeding mode, faecal deposition and sediment depth penetration (Macoma balthica, Cerastoderma glaucum and Mya arenaria) were selected as model organisms: our experiment demonstrated that although the bivalves had no overall effect on total meiobenthic densities, they variously affected a number of meiobenthic major taxa. Species buried under the sediment surface (M. balthica, M. arenaria) facilitated meiobenthos to penetrate deeper sediment layers compared to the surface-dwelling bivalves (C. glaucum). The most conspicuous response of meiobenthos and nematodes to bivalve activities was, however, observed at the sediment surface: densities of juvenile bivalves and rotifers, and the dominant nematode species (Adoncholaimus thalassophygas, Desmolaimus cf. zeelandicus and Halomonhystera disjuncta) were significantly reduced in surface sediments in Macoma microcosms compared to other treatments. It is suggested that different bivalve feeding modes (deposit-feeding vs. suspension-feeding), attributes of faecal deposition (to the sediment surface vs. to subsurface sediments) and depth of sediment penetration can significantly change meiobenthic and nematode communities in sandy sediments.

Combined effects of drought and the fungicide tebuconazole on aquatic leaf litter decomposition

Loss of biodiversity and altered ecosystem functioning are driven by the cumulative effects of multiple natural and anthropogenic stressors affecting both quantity and quality of water resources. Here we performed a 40-day laboratory microcosm experiment to assess the individual and combined effects of drought and the model fungicide tebuconazole (TBZ) on leaf litter decomposition (LLD), a fundamental biogeochemical process in freshwater ecosystems. Starting out from a worst-case scenario perspective, leaf-associated microbial communities were exposed to severe drought conditions (four 5-day drought periods alternated with 4-day immersion periods) and/or a chronic exposure to TBZ (nominal concentration of 20μgL−1). We assessed the direct effects of drought and fungicide on the structure (biomass, diversity) and activity (extracellular enzymatic potential) of fungal and bacterial assemblages colonizing leaves. We also investigated indirect effects on the feeding rates of the amphipod Gammarus fossarum on leaves previously exposed to drought and/or TBZ contamination. Results indicate a stronger effect of drought stress than fungicide contamination under the experimental conditions applied. Indeed, the drought stress strongly impacted microbial community structure and activities, inhibiting the LLD process and leading to cascading effects on macroinvertebrate feeding. However, despite the lack of significant effect of TBZ applied alone, the effects of drought on microbial functions (i.e., decrease in LLD and in enzymatic activities) and on Gammarus feeding rates were more pronounced when drought and TBZ stresses were applied together. In a perspective of ecological risk assessment and ecosystem management for sustainability, these findings stress the need for deeper insight into how multiple stressors can affect the functioning of aquatic ecosystems and associated services.

The Earthworm Eisenia fetida Can Help Desalinate a Coastal Saline Soil in Tianjin, North China.

A laboratory microcosm experiment was conducted to determine whether the earthworm Eisenia fetida could survive in a saline soil from a field site in North China, and an experiment using response surface methodology was conducted at that field site to quantify the effects of E. fetida and green waste compost (GWC) on the salt content of the soil. The microcosm results showed that E. fetida survived in GWC-amended saline soil and increased the contents of humic acid, available N, and available P in the GWC-amended soil. The data from the field experiment were described by the following second-order model:, where y is the decrease in soil salinity (g of salt per kg of dry soil) relative to the untreated control, x 1 is the number of E. fetida added per m2, and x 2 is the quantity of GWC added in kg per m2. The model predicted that the total salt content of the saline soil would decrease by > 2 g kg-1 (p<0.05) when 29–90 individuals m-2 of E. fetida and 6.1–15.0 kg m-2 of GWC were applied. We conclude that the use of E. fetida for soil desalination is promising and warrants additional investigation.

Co-occurrence of Photochemical and Microbiological Transformation Processes in Open-Water Unit Process Wetlands.

The fate of anthropogenic trace organic contaminants in surface waters can be complex due to the occurrence of multiple parallel and consecutive transformation processes. In this study, the removal of five antiviral drugs (abacavir, acyclovir, emtricitabine, lamivudine and zidovudine) via both bio- and phototransformation processes, was investigated in laboratory microcosm experiments simulating an open-water unit process wetland receiving municipal wastewater effluent. Phototransformation was the main removal mechanism for abacavir, zidovudine, and emtricitabine, with half-lives (t1/2,photo) in wetland water of 1.6, 7.6, and 25 h, respectively. In contrast, removal of acyclovir and lamivudine was mainly attributable to slower microbial processes (t1/2,bio = 74 and 120 h, respectively). Identification of transformation products revealed that bio- and phototransformation reactions took place at different moieties. For abacavir and zidovudine, rapid transformation was attributable to high reactivity of the cyclopropylamine and azido moieties, respectively. Despite substantial differences in kinetics of different antiviral drugs, biotransformation reactions mainly involved oxidation of hydroxyl groups to the corresponding carboxylic acids. Phototransformation rates of parent antiviral drugs and their biotransformation products were similar, indicating that prior exposure to microorganisms (e.g., in a wastewater treatment plant or a vegetated wetland) would not affect the rate of transformation of the part of the molecule susceptible to phototransformation. However, phototransformation strongly affected the rates of biotransformation of the hydroxyl groups, which in some cases resulted in greater persistence of phototransformation products.

Enhanced and stabilized arsenic retention in microcosms through the microbial oxidation of ferrous iron by nitrate.

Magnetite strongly retains As, and is relatively stable under Fe(III)-reducing conditions common in aquifers that release As. Here, laboratory microcosm experiments were conducted to investigate a potential As remediation method involving magnetite formation, using groundwater and sediments from the Vineland Superfund site. The microcosms were amended with various combinations of nitrate, Fe(II) (aq) (as ferrous sulfate) and lactate, and were incubated for more than 5 weeks. In the microcosms enriched with 10 mM nitrate and 5 mM Fe(II) (aq), black magnetic particles were produced, and As removal from solution was observed even under sustained Fe(III) reduction stimulated by the addition of 10 mM lactate. The enhanced As retention was mainly attributed to co-precipitation within magnetite and adsorption on a mixture of magnetite and ferrihydrite. Sequential chemical extraction, X-ray absorption spectroscopy and magnetic susceptibility measurements showed that these minerals formed at pH 6-7 following nitrate-Fe(II) addition, and As-bearing magnetite was stable under reducing conditions. Scanning electron microscopy and X-ray diffraction indicated that nano-particulate magnetite was produced as coatings on fine sediments, and no aging effect was detected on morphology over the course of incubation. These results suggest that a magnetite based strategy may be a long-term remedial option for As-contaminated aquifers.

Soil, slurry and application effects on greenhouse gas emissions

In conjunction with soil type and climate, the use of nitrogen fertilizers is a major factor affecting nitrous oxide emissions. This study compares injection of pig slurry and pig slurry digestate at 15 or 20 cm depths with trailing-hose application followed by immediate incorporation. The work was based on a laboratory microcosm experiment with undisturbed soil cylinders (0-30 cm depth) from three arable soils (Histosol, Gleysol and Plaggic Anthrosol). Soil cylinders were fertilized with pig slurry and pig slurry digestate (150 kg N/ha) and emissions of N2O, CH4 and CO2 were monitored. The comparison of application techniques over a 37-day period show that soil type and application technique, had a strong (P < 0.001) impact on N2O emissions. Fertilization with pig slurry showed no significantly higher N2O emissions than pig slurry digestate. Fertilizer injection significantly increased N2O emissions compared to fertilization with a trailing-hose with incorporation.

Differential Decay of Wastewater Bacteria and Change of Microbial Communities in Beach Sand and Seawater Microcosms.

Laboratory microcosm experiments were conducted to determine the decay kinetics of wastewater bacteria and the change of microbial communities in beach sand and seawater. Cultivation-based methods showed that common fecal indicator bacteria (FIBs; Escherichia coli, enterococci, and Clostridium perfringens) exhibited biphasic decay patterns in all microcosms. Enterococci and C. perfringens, but not E. coli, showed significantly smaller decay rates in beach sand than in seawater. Cultivation-independent qPCR quantification of 16S rRNA gene also showed significantly slower decrease of total bacterial densities in beach sand than in seawater. Microbial community analysis by next-generation sequencing (NGS) further illustrated that the decreasing relative abundance of wastewater bacteria was contrasted by the increase in indigenous beach sand and seawater microbiota, and the overall microbial community dynamics corresponded well with the decay of individual FIB populations. In summary, the differential decay of wastewater bacteria in beach sand and in seawater provides a kinetic explanation to the often-observed higher abundance of FIBs in beach sand, and the NGS-based microbial community analysis can provide valuable insights to understanding the fate of wastewater bacteria in the context of indigenous microbial communities in natural environments.

Experimental Modeling of Cyanobacterial Bloom in a Thermokarst Lake: Fate of Organic Carbon, Trace Metal, and Carbon Sequestration Potential

Thermokarst lakes, formed during permafrost thaw in Western Siberia Plain over past tens to hundreds years, cover overall territory close to million km2 and may represent significant source of CO2 and CH4 to the atmosphere. These acidic (3 < pH < 6) and humic [10 < dissolved organic carbon (DOC) < 50 mg/L] lakes are essentially inhabited by heterotrophic bacterioplankton with rare phytoplankton bloom occurring during warm periods. In order to understand possible effects of phytoplankton bloom on thermokarst lake hydrochemistry under climate warming scenario, we cultured pure cyanobacterium (Gloeocapsa sp.) and native cyanobacterial associate separated from the natural lake water. As substrates, sterilized thermokarst lake water and peat leachate from western Siberia were used. In these laboratory microcosm experiments which lasted 10 days, we monitored daily pH, biomass, DOC, and 40 major and trace elements. Despite significant variation of pH (4 to ~10.5) and biomass (a factor of 3–5), very few dissolved elements responded to massive cyanobacterial growth. The DOC varied within a factor of 1.2–1.5, exhibiting slow increase due to exometabolite production in thermokarst lake water and an initial decrease due to photodegradation in peat leachate. Elements appreciably affected by photosynthesis in both lake water and peat leachate substrates were P, Zn, Mn, and, in a lesser degree, Cd, K, Rb, Sr, Ba, Cr, Al, and U. While P, K (Rb), Mn, and Zn removal from solution during cell growth could be linked to biological demand by cyanobacteria, the adsorption of Cd, Sr, Ba, Al, Cr, U on the cell surface in response to the pH rise is most likely. Many other trace elements did not exhibit any significant evolution of the concentration during 10-day experiment either due to their strong complexation with allochthonous organic matter and essentially organic/organo-mineral colloidal status (Fe, Ni, Co, Cu, Pb, REEs, Ti, Zr, Hf, Th) or due to the lack of element interaction with cyanobacterial cells, via both adsorption and intracellular uptake (B, Si, V, Mo, As, Sb, Cs). Therefore, possible intensification of cyanobacterial bloom in thermokarst lakes caused by leaching of thawing peat will likely affect only few macronutrients and micronutrients such as P, K, Mn, and Zn, while the majority of trace elements bound to allochthonous DOC in the form of organic and organo-mineral colloids will not be affected by cyanobacterial biomass production and pH rise due to photosynthesis. Cyanobacterial bloom in organic-rich (20 mg DOC/L) thermokarst lakes exhibited significant potential of carbon sequestration from the atmosphere, which is more than an order of magnitude higher than the CO2 evasion due to heterotrophic plankton respiration of allochthonous DOC.

Ecological effects of Irgarol 1051 and Diuron on a coastal meiobenthic community: A laboratory microcosm experiment

After the Tributyltin world ban, Irgarol 1051 and Diuron have been the most commonly used biocides in antifouling paints. When adsorbed to suspended particulate matter or introduced as paint particles, these compounds accumulate in marine sediments and potentially cause ecological damage to benthic organisms. Therefore, a microcosm experiment was designed to evaluate the effects of Irgarol 1051 and Diuron, individually, on a meiofaunal community with emphasis on the dominant nematode assemblages. The experiment tested two factors: “Treatment” (two types of controls and three environmentally relevant concentrations of each contaminant) and “Exposure time” (5, 15 and 30 days). Significant declines in meiofauna density, nematode species richness and diversity, and changes in multivariate community structure were observed for both biocides at all exposure levels when compared to controls. Decreases occurred early on, within five days of exposure, which suggests that mortality, and not sub-lethal effects, has befallen upon the organisms. Sediment chlorophyll a and pheopigment concentrations, and redox potential were monitored to verify any indirect effects to the fauna through changes in the environment and results gave no indications of such mediated effects pointing to a direct toxic effect of both Irgarol and Diuron on the meiofauna. Although contaminated treatments showed a significant decline in the relative abundances of a particular functional group represented by the larger, longer-lived species, we did not observe the typical expected switch to smaller, more opportunistic taxa. Indeed, differences between controls and contaminated treatments were mainly due to an overall reduction in densities of the most abundant species in contaminated treatments. The high mortality (ca. 50% decline in total abundances), changes in community structure and species loss observed at biocide levels frequently encountered in the field suggest Irgarol and Diuron as a threat to benthic communities. Such severe effects contrast to other studies that have detected lower impacts, suggesting the free-living nematodes as potential indicators of marine pollution and the microcosm approach using natural communities as an impending tool for ecotoxicological studies.

Potential impact of Chironomus plumosus larvae on hypolimnetic oxygen in the central basin of Lake Erie

Previous studies have indicated that burrow-irrigating infauna can increase sediment oxygen demand (SOD) and impact hypolimnetic oxygen in stratified lakes. We conducted laboratory microcosm experiments and computer simulations with larvae of the burrowing benthic midge Chironomus plumosus to quantify burrow oxygen uptake rates and subsequent contribution to sediment oxygen demand in central Lake Erie. Burrow oxygen uptake and water flow velocities through burrows were measured using oxygen microelectrodes and hot film anemometry, respectively. Burrow oxygen consumption averaged 2.66 × 10− 10 (SE = ± 7.82 × 10− 11) mol O2/burrow/s at 24 °C and 9.64 × 10− 10 (SE = ± 4.86 × 10− 10) mol O2/burrow/s at 15 °C. In sealed microcosm experiments, larvae increased SOD 500% at 24 °C (density = 1508/m2) and 375% at 15 °C (density = 864/m2). To further evaluate effects of densities of C. plumosus burrows on SOD we developed a 3-D transport reaction model of the process. Using experimental data and chironomid abundance data in faunal surveys in 2009 and 2010, we estimated that bioirrigation by a population of 140 larvae/m2 could account for between 2.54 × 10− 11 mol/L/s (model results) and 5.58 × 10− 11 mol/L/s (experimental results) of the average 4.22 × 10− 11 mol/L/s oxygen depletion rate between 1970 and 2003, which could have accounted for 60–132% of the oxygen decline. At present, it appears that the population density of this species may be an important factor in development of hypoxic or anoxic conditions in central Lake Erie.

Chemical Oxidation Performance in High Temperature, Saline Groundwater Impacted With Hydrocarbons

A series of laboratory microcosm experiments and a field pilot test were performed to evaluate the potential for in situ chemical oxidation (ISCO) of aromatic hydrocarbons and methyl tertiary butyl ether (MTBE), a common oxygenate additive in gasoline, in saline, high temperature (more than 30 °C) groundwater. Groundwater samples from a site in Saudi Arabia were amended in the laboratory portion of the study with the chemical oxidants, sodium persulfate (Na2S2O8) and sodium percarbonate (Na2(CO3)2), to evaluate the changes in select hydrocarbon and MTBE concentrations with time. Almost complete degradation of the aromatic hydrocarbons, naphthalene and trimethylbenzenes (TMBs), was found in the groundwater sample amended with persulfate, whereas the percarbonate‐amended sample showed little to no degradation of the target hydrocarbon compounds in the laboratory. Isotopic analyses of the persulfate‐amended samples suggested that C‐isotope fractionation for xylenes occurred after approximately 30 percent reduction in concentration with a decline of about 1 percent in the δ13C values of xylenes. Based on the laboratory results, pilot‐scale testing at the Saudi Arabian field site was carried out to evaluate the effectiveness of chemical oxidation using nonactivated persulfate on a high temperature, saline petroleum hydrocarbon plume. Approximately 1,750 kg of Na2S2O8 was delivered to the subsurface using a series of injection wells over three injection events. Results obtained from the pilot test indicated that all the target compounds decreased with removal percentages varying between 86 percent for naphthalene and more than 99 percent for the MTBE and TMBs. The benzene, toluene, ethylbenzene, and xylene compounds decreased to 98 percent on average. Examination of the microbial population upgradient and downgradient of the ISCO reactive zone suggested that a bacteria population was present following the ISCO injections with sulfate‐reducing bacteria (SRB) being the dominant bacteria present. Measurements of inorganic parameters during injection and postinjection indicated that the pH of the groundwater remained neutral following injections, whereas the oxidation–reduction potential remained anaerobic throughout the injection zone with time. Nitrate concentrations decreased within the injection zone, suggesting that the nitrate may have been consumed by denitrification reactions, whereas sulfate concentrations increased as expected within the reactive zone, suggesting that the persulfate produced sulfate. Overall, the injection of the oxidant persulfate was shown to be an effective approach to treat dissolved aromatic and associated hydrocarbons within the groundwater. In addition, the generation of sulfate as a byproduct was an added benefit, as the sulfate could be utilized by SRBs present within the subsurface to further biodegrade any remaining hydrocarbons. ©2015 Wiley Periodicals, Inc.

Resource niche overlap promotes stability of bacterial community metabolism in experimental microcosms.

Decomposition of organic matter is an important ecosystem process governed in part by bacteria. The process of decomposition is expected to benefit from interspecific bacterial interactions such as resource partitioning and facilitation. However, the relative importance of resource niche breadth (metabolic diversity) and resource niche overlap (functional redundancy) on decomposition and the temporal stability of ecosystem processes received little scientific attention. Therefore, this study aims to evaluate the effect of an increase in bacterial community resemblance on both decomposition and the stability of bacterial metabolism in aquatic sediments. To this end, we performed laboratory microcosm experiments in which we examined the influence of bacterial consortia differing in number and composition of species on bacterial activity (Electron Transport System Activity, ETSA), dissolved organic carbon production and wavelet transformed measurements of redox potential (Eh). Single substrate affinities of the individual bacterial species were determined in order to calculate the metabolic diversity of the microbial community. Results presented here indicate that bacterial activity and organic matter decomposition increase with widening of the resource niche breadth, and that metabolic stability increases with increasing overlap in bacterial resource niches, hinting that resource niche overlap can promote the stability of bacterial community metabolism.

Functional dissimilarity across trophic levels as a driver of soil processes in a Mediterranean decomposer system exposed to two moisture levels

The role of biodiversity for soil processes remains poorly understood. Existing evidence suggests that functional diversity rather than species richness is relevant for soil functioning. However, the importance of functional diversity has rarely been assessed simultaneously at more than one trophic level, critically limiting the prediction of consequences of biodiversity loss for soil functioning. In a laboratory microcosm experiment, we tested the hypothesis that increasing functional dissimilarity of both litter‐feeding soil fauna and litter mixtures interactively affects the rates of five different soil processes related to litter decomposition. We created trait‐based functional dissimilarity gradients using five assemblages of two detritivore species and five mixtures of two plant litter species commonly found in Mediterranean shrubland ecosystems of southern France. With increasing drought periods predicted for Mediterranean ecosystems in the future, we additionally included two different watering frequencies to evaluate the impact of drought on soil processes and how drought interacts with functional dissimilarity. The different fauna assemblages and litter mixtures showed strong effects on litter mass loss, soil organic carbon and nitrogen leaching, as well as on soil microbial activities. Up to 20% of the variation in response variables was explained by functional dissimilarity, suggesting an ecologically relevant impact of functional diversity on soil process rates. Detritivore functional dissimilarity tended to have stronger effects when combined with increasingly dissimilar litter mixtures, suggesting that trait dissimilarity interacts across trophic levels. Drought affected several soil processes but did not modify the relationships between functional dissimilarity and process rates. Our results indicate that trait diversity of detritivore assemblages and litter mixtures is an important predictor of soil process rates. The common and easily measurable traits used in our study suggest straightforward application across different types of ecosystems and environmental conditions.

DEGRADATION AND ADSORPTION BEHAVIOR OF DIBUTYL PHTHALATE IN METHANOGENIC PHASE REFUSE

The degradation and adsorption behavior of dibutyl phthalate (DBP) in methanogenic phase refuse was investigated through laboratory microcosm experiments. The results showed that the half-life of DBP in the sterilized refuse was 5.9 times higher than in unsterilized samples, but that it decreased by 35.8% when dominant bacterial strains were added. Different concentrations of DBP did not have obvious effects on its degradation. The half-lives of DBP were decreased by 53.0%, 37.2% and 20.8% when the refuse moisture increased from 20%, 40% and 60% to 80%, respectively. The pH of refuse was an important factor influencing DBP biodegradation, with the optimal pH being around 7.0. The optimal temperature for DBP degradation in refuse was around 30°C. In addition, the Freundlich model fits the adsorption and desorption isotherm of DBP for refuse with n values that suggest nonlinear adsorption characteristics. The free energy change ΔG value (−23.5 kJ mol −1 ) indicates that the adsorption of DBP on refuse was a physical reaction. Desorption hysteresis was observed in the DBP desorption experiments. Overall, the results indicate that DBP may accumulate in refuse, and that its transformation and bio-availability may be limited under landfill conditions.

Out with the old, in with the new? Younger Daphnia clones are competitively superior over centuries-old ancestors

A laboratory microcosm experiment was conducted to examine the roles of food quantity and food quality on the competitive abilities of clones of the keystone aquatic zooplankter, Daphnia pulicaria. Using methods of resurrection ecology, clones were established by hatching dormant eggs from sediment layers of a lake (South Center, MN, U.S.A) that were separated by centuries of environmental change (nutrient enrichment). Two sets of paired clones (“modern” vs. “ancient”) were competed against each other in a 2 × 2 factorial design of high/low food quantity and high/low food quality. Experimental carbon:phosphorus (C:P) ratios were designed to mimic food quantity and food quality parameters respective to each of the two time periods from which the clones were extracted to test whether clones are competitively superior under conditions representative of the time period in which they existed. Contrary to predictions, we found that “modern” clones were able to outcompete “ancient” clones under all food treatments. We discuss potential mechanisms (i.e., mutational input, viability of long-dormant eggs, differences in phenotypic plasticity) that may have differentially impacted the performance of experimental clones. Our results should aid in deciphering microevolutionary dynamics observed in resurrection ecology studies, a powerful tool that can illuminate adaptive dynamics of organisms to environmental changes on long-term (centuries-long) temporal scales.

Anaerobic nitrification–denitrification mediated by Mn-oxides in meso-tidal sediments: Implications for N2 and N2O production

Field measurements in the Arcachon Bay (southwest France) indicated anaerobic production of NOx via nitrification, which was coupled to the reduction of Mn-oxides. To prove the occurrence of this process, laboratory microcosm experiments were set up. A 15N tracer-based approach was used to track if NOx produced through Mn-oxide-mediated anaerobic nitrification would be reduced to N2 via denitrification or anammox. We also hypothesized the generation of the potent greenhouse gas nitrous oxide (N2O) during nitrification–denitrification in the presence of Mn-oxides. The microcosms were prepared using sediment sectioned at varying depths (0–2.5, 2.5–4.5, 4.5–8.5, 8.5–12 and 12–17cm) during two sampling campaigns in October (fall) and January (winter). Labeling with 15NO3− revealed low N2 production originating from NO3− in the water column (Pw), which did not increase significantly on amendment with Mn-oxides during both sampling periods. However, for both seasons, a significant increase of N2 produced via nitrification (Pn) was observed upon addition of Mn-oxides reaching 76-fold enhancement at ≤2.5cm. To support these results, sediment slurries of October were subjected to amendment of 15NH4+, 14NO3− with or without addition of Mn-oxides. A substantial production of P15 (N2 production from 15NH4+) within 0–17cm provided further evidence on nitrification–denitrification mediated by Mn-oxides probably with minimal intervention of anammox. In organically rich sediments, anaerobic nitrification–denitrification mediated by Mn-oxides could play an important role in lowering re-mineralized NH4+ levels in the benthic system. As hypothesized, significant production of N2O through the pathway was observed revealing newer mechanisms leading to the generation of the radiative gas.

Evolutionary history and the strength of species interactions: testing the phylogenetic limiting similarity hypothesis.

A longstanding concept in community ecology is that closely related species compete more strongly than distant relatives. Ecologists have invoked this "limiting similarity hypothesis" to explain patterns in the structure and function of biological communities and to inform conservation, restoration, and invasive-species management. However, few studies have empirically tested the validity of the limiting similarity hypothesis. Here we report the results of a laboratory microcosm experiment in which we used a model system of 23 common, co-occurring North American freshwater green algae to quantify the strength of 216 pairwise species' interactions (the difference in population density when grown alone vs. in the presence of another species) along a manipulated gradient of evolutionary relatedness (phylogenetic distance, as the sum of branch lengths separating species on a molecular phylogeny). Interspecific interactions varied widely in these bicultures of phytoplankton, ranging from strong competition (ratio of relative yield in polyculture vs. monoculture << 1) to moderate facilitation (relative yield > 1). Yet, we found no evidence that the strength of species' interactions was influenced by their evolutionary relatedness. There was no relationship between phylogenetic distance and the average, minimum (inferior competitor), nor maximum (superior competitor) interaction strength across all biculture communities (respectively, P = 0.19, P = 0.17, P = 0.14; N = 428). When we examined each individual species, only 17% of individual species' interactions strengths varied as a function of phylogenetic distance, and none of these relationships remained significant after Bonferoni correction for multiple tests (N = 23). Last, when we grouped interactions into five qualitatively different types, the frequency of these types was not related to phylogenetic distance among species pairs (F4,422 = 1.63, P = 0.15). Our empirical study adds to several others that suggest the biological underpinnings of competition may not be evolutionarily conserved, and thus, ecologists may need to re-evaluate the previously assumed generality of the limiting similarity hypothesis.